1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, a secondary battery in a pouch casing with a reinforcement structure.
2. Description of the Related Art
Secondary batteries are rechargeable and can be made in small sizes with high capacities. Typical examples of secondary batteries include nickel-metal hydride (Ni-MH) batteries and lithium (Li) secondary batteries.
Secondary batteries are classified into cylindrical batteries using a cylindrical aluminum can, rectangular batteries using a rectangular aluminum can, and pouch type batteries where the battery is accommodated in a thin-film pouch type casing.
A pouch type secondary battery, as shown in FIG. 1, includes an electrode assembly 10 which generates a predetermined current, and a pouch casing 20 which encloses and hermetically seals the electrode assembly 10. The electrode assembly 10 has an electrode tab 14 drawn from one side which is connected with a lead 15 that serves as an electrical path leading outside. The lead 15 has a protective tape 16 adhered to its surface so that it extends outside the pouch casing 20.
The pouch casing 20 includes a case body 22 having an internal space in which the electrode assembly 10 is retained, and a cover 24 integrally extending from one end of the case body 22. A flange 23 is formed at the periphery of the case body 22 and is configured to be welded with the cover 24 to hermetically seal the case body 22.
The pouch casing 20 of the pouch type secondary battery having the above-described configuration is typically made of a metal foil and resin materials. FIG. 2 is a cross-sectional view taken along the line I-I shown in FIG. 1. As shown in FIG. 2, the pouch casing 20 is formed of composite materials compressed such that a thermal adhesion layer 22b is formed on the inner face of a foil 22a. The thermal adhesion layer 22b may be formed from denatured polypropylene, e.g., cast polypropylene (CPP). The foil 22a may be made of a metal, e.g., Al. A sheathing 22c made of a polymer resin, e.g., nylon or polyethyleneterephthalate (PET), is formed on the outer face of the foil 22a. The thermal adhesion layer interposed between the foil 22a and the sheathing 22c. Thus, the pouch casing 20 can be sealed by heat compression in a state in which thermal adhesion layers of the case body 22 and the pouch cover 24 are joined to each other.
However, since the pouch casing having the above-described configuration is very thin, there is a limitation in protecting the electrode assembly housed in the pouch casing. In particular, in such a pouch casing, the thickness of a metal foil provided for the purpose of maintaining the strength of the casing is only several tens of micrometers. Accordingly, the metal foil is very weak against external impacts.
When a battery using the conventional pouch casing is cut or shocked by a sharp tool, the safety of the battery may be impaired, resulting in poor reliability.
In such a pouch casing, the melting point of the thermal adhesion layer made of, for example, CPP, is about 130° C. to 140° C., and the melting point of a separator of the electrode assembly housed in the pouch casing is also about 130° C. to 140° C. The temperature of thermally welding a case body and a pouch cover of the pouch casing is about 180° C. to 210° C. Thus, during thermal welding of the pouch, the thermal adhesion layer and the separator may melt, or the separator may be fusibly fixed with the thermal adhesion layer. Also, the thermal adhesion layer melts exposing the Al metal film inside, so that the Al metal film may be short-circuited from the electrode assembly.
In a lithium polymer secondary battery, a stacked electrode assembly is housed in the pouch casing and has a bi-cell structure, as shown in FIG. 3. The bi-cell structure is a unit of an electrode assembly, and is generally constructed such that a positive electrode sheet 11 is stacked on both faces of a negative electrode sheet 12 with a separator 13 interposed between each of the positive and negative electrode sheets 11 and 12. As shown in FIG. 3, the area of each separator 13 defining the positive and negative electrode sheets 11 and 12 is greatest and the area of the positive electrode sheet 11 is smallest. A plurality of bi-cell laminates is stacked to form an electrode assembly 10.
As described above, when the electrode assembly 10 having a plurality of bi-cell laminates stacked is housed in a pouch casing, the battery swells due to gas generated inside the battery as the battery is repeatedly charged and discharged. When gas is generated inside electrode sheets of the electrode assembly, the electrode sheets may swell in a direction indicated by an arrow shown in FIG. 3, while the separator 13 shrinks somewhat. Accordingly, the positive electrode sheet 11 and the negative electrode sheet 12 may be short-circuited from each other at their edges.
Therefore, it is necessary to protect the battery from short-circuit due to swelling of the electrode assembly.
U.S. Pat. No. 6,042,966 discloses a battery structure for preventing electrical short between an electrode tab exposed outward and a cut edge of a pouch, wherein the pouch comprises an outer lamination layer made of a packaging insulator film on the outer face of a metal foil, and an inner lamination layer having a polyamide layer made of, for example, nylon, and a thermal adhesion layer sequentially formed on the inner face of the metal foil.
U.S. Pat. No. 6,106,973 discloses a pouch formed by folding a laminated film having an Al film at the inner side and a polypropylene film at the outer side on both sides of an electrode assembly.
The above-described pouches having thin films still have several drawbacks including poor structural stability. Also, in the case of using a stacked electrode assembly, short-circuit between electrodes may occur due to swelling of the electrode assembly.